This invention relates to apparatus for sealing joints of a steam turbine shell and, more particularly, to apparatus for sealing wherein bolting forces are inadequate to ensure an efficient seal, and further wherein the apparatus may be disposed skewed with respect to the axis of rotation of the rotor of the turbine.
In general, steam turbines operate to convert energy stored in high-pressure, high-temperature steam into rotational mechanical movement. Steam turbines employed by electric utilities in the generation of electric power, typically comprise a plurality of turbine blades, or buckets, radially mounted on the periphery of a rotor shaft and disposed so as to form a plurality of bucket wheels. The rotor shaft, with associated bucket wheels, is mounted on bearings with the bucket wheels disposed inside an inner shell which is in turn surrounded by a spaced apart outer shell. This double shell configuration forms a pressurizable housing in which the bucket wheels rotate and prevents potentially damaging thermal gradients. The bucket wheels are disposed between stationary nozzle rings which are formed by circular arrays of stationary curved partitions substantially radially disposed between and fixedly retained by a pair of concentric diaphragm rings. These partitions are generally referred to as nozzle partitions and the spaces between the partitions as nozzles. As steam flows through the interior cavity of the pressurizable inner shell, it alternately passes through sequences of stationary nozzle partitions and rotating turbine bucket wheels to produce rotational movement of the rotor shaft. The combination of a pair of diaphragm rings with their associated partitions and the cooperating row of buckets is generally referred to as a stage, stages being numbered sequentially in the direction of steam flow starting from the steam input region. These concepts are elementary and are generally well known in the turbine art.
Modern large steam turbines generally comprise several sections such as, for example, high-pressure, intermediate pressure, low-pressure and reheat. These sections possess various design characteristics so as to permit extraction of the optimum amount of energy from the expansion of steam through the respective turbine sections, thereby optimizing overall turbine efficiency. It is common practice to have one or more of these sections configured in a double flow arrangement, in which steam entering a middle portion, or tub, of the section encounters a diverging flow path. After entry into this middle portion of one of the turbine sections, steam exits in substantially opposite directions, wherein the oppositely directed steam flows are used to impart rotation in the same direction to the turbine shaft. Thus, for example, steam entering from the top or bottom of a turbine section having a horizontally disposed rotor exits toward the left and right to flow generally axially through the turbine. This double flow configuration beneficially contributes to overall machine efficiency.
The inner shell of a double flow reheat section of a turbine may be configured to provide an annular chamber for receiving and circumferentially distributing steam entering the reheat section. The inner wall of the annular chamber is configured to direct steam into the entrance region for the opposing nozzle partitions of the first stage of the reheat section. Typically the inner shell may be configured in axially extending sections, such as halves, for ease of construction and assembly. One half is generally disposed such that when the other half is abutted to it, the joint between the halves is substantially horizontal. Although the apparatus of the present invention will be described with respect to its application to a horizontal joint, it is to be understood that the apparatus of the present invention is not so limited and may be used for sealing a joint regardless of the spatial orientation of the joint. In addition, for ease of manufacture, generally a major portion of the inner shell is a casting. However, the apparatus of the present invention may be used for sealing a joint between portions of turbine components, regardless of the methods employed to fabricate the portions of components.
The mating horizontal joint surfaces of the inner shell may be tightly secured to each other by providing a bolting flange along the outer periphery of the joint. However, there is a maximum distance from a bolt, which distance depends in part on the material being secured and the operating environment, beyond which sealing contact cannot be ensured. The radially inner wall at the flange of the annular chamber, for circumferentially distributing input steam to the reheat section and defined by the inner shell, is generally too far displaced from any bolt disposed through the bolting flange to ensure sealing contact between another mateable flange and the radially inner wall of the annular chamber from another section of the inner shell. Further, the inner shell is subject to a temperature differential across it due to operating parameters of the steam. For example, inlet steam temperature may be about 1000.degree. F. (which is also the steam temperature in the annular chamber) and the temperature of steam around the outer periphery of the inner shell, i.e. in the cavity between the inner and outer shell, may be about 800.degree. F. This temperature differential across the mateable joint surfaces causes unequal circumferential elongation or expansion across the sealing flange and inner wall of the annular chamber. The inner wall of the annular chamber is generally disposed beyond the maximum efficient sealing distance from any flange bolts and may circumferentially expand more than the outer wall of inner shell, even though it has a smaller circumference, resulting in a potential gap or steam leakage path between the steam inlet chamber and annular chamber, and the volume between the inner and outer shell. Another potential steam leakage path is between the annular chamber and the second stage of the reheat section (wherein steam pressure is less than at the input to the first stage, since it has already been expanded through the first stage).
Prior attempts to seal joints against the undesirable passage of steam have employed a key disposed in a keyway wherein a part of the keyway was formed into each of the mating sealing faces. Because of the inherent nature of machines necessary for fabricating such keyways and the close tolerances required in order to prevent eventual steam leakage, it is very difficult to form such a keyway so that mating portions of the keyway will register exactly, especially when it is required that the keyway be skewed with respect to the rotational axis of the turbine. Improper mating and registration of respective keyway portions may result in an ineffective steam seal and eventual steam leakage.
Accordingly, it is an object of the present invention to provide sealing apparatus for a joint of a steam turbine shell wherein registratable mating keyway portions in each section of the mating faces of the joint are eliminated.
Another object of the present invention is to provide sealing apparatus for a joint of a steam turbine shell which may be skewed with respect to the rotational axis of the steam turbine.